Field of the Invention
The present invention concerns methods and systems for magnetic resonance (MR) imaging, and in particular to such methods and systems that make use of a frequency-modulated balanced steady-state free precession (FM bSSFP) sequence in order to acquire raw data from a subject, with the raw data then being used to reconstruct an image of the region of the subject from which the raw data were acquired.
Description of the Prior Art
In order to acquire all data that are necessary so as to reconstruct a magnetic resonance image, it is necessary for all data entry locations in k-space to be filled, i.e., a data entry must be made at each available point in k-space. Typically, this requires several excitations of nuclear spins in the region of interest in the subject, and readout of the resulting MR signals along different trajectories in k-space. Typical examples of such trajectories are parallel readout lines in k-space in Cartesian imaging, and radial lines in k-space in projection reconstruction imaging.
Balanced steady-state free precession (bSSFP) sequences are known and often used in MR imaging due to their short scan times, high SNR and excellent contrast. A drawback of such sequences is that they are prone to banding artifacts in the reconstructed image, which can considerably reduce the image quality. Such problems are described in Schär et al. “SSFP Imaging at 3 T Tesla,” Magnetic Resonance in Medicine, Vol. 51, pp. 799-806 (2004) and Bangerter et al., “Analysis of Multiple-Acquisition SSFP,” Magnetic Resonance in Medicine, Vol. 51, pp. 1038-1047 (2004).
The article Foxall et al., “Frequency-Modulated Steady-State Free Precession Imaging,” Magnetic Resonance in Medicine, Vol. 48, pp. 502-508 (2002) demonstrates that the steady state of bSSFP does, in fact, tolerates slow frequency changes, thereby enabling a sweep to be made through different frequencies in one acquisition. The dynamic range of frequency from one acquired line of k-space to another, within one radial measurement, has proven suitable to create banding-free images, even in the presence of high-field inhomogeneities. This has been presented in Benkert et al., “Dynamically Phase-Cycled Radial Balanced SSFP Imaging for Efficient Banding Removal,” Magnetic Resonance in Medicine, Vol. 73, pp. 182-194 (2015) and Benkert et al., “Fast Isotropic Banding-Free bSSFP Imaging Using 3D Dynamically Phase-Cycled Radial bSSFP (3D DYPR SSFP), Z. für Med. Phys., Vol. 26, pp. 63-74 (2016).
In the final image, it can be of interest to determine whether the signal of a certain pixel originated in tissue containing primarily fat or containing primarily water. Particularly in bSSFP, fat tissue produces a very bright signal in comparison to water, which can obscure underlying pathologies. This has been noted in Wansapura, “Abdominal Fat-Water Separation with SSFP at 3 T Tesla,” Pediatric Radiology, Vol. 37, pp. 68-73 (2006).